She was found just outside the perimeter of the park, near a road. "Trained keepers attempted to tranquilize her but, unfortunately, she was out of range," the zoo said in a statement. "Therefore, the difficult decision was made to euthanize the animal."

"At no point during this incident were any of our visitors in any danger. The safety and well-being of all our visitors is our first priority," the zoo said in a follow-up statement. "We are confident that this incident was an isolated case and that the replacement equipment, combined with an even more intensive electric fence-testing regime, will ensure that our wolf enclosure will provide a safe and secure home for our wolves."

Ember's 10-week-old pups, who are the first pups to be born at the zoo in its 47-year history, are left without their mother, but their father, Ash, appears to be taking on full responsibility for the pups.

"Ash, our male wolf, is displaying encouraging ‘natural’ behavior as a single parent to his 10-week-old cubs, who are close to being fully weaned," the zoo said. "We must also bear in mind that Ash is young and this is his first litter. However, we remain confident that the cubs will continue to grow from strength to strength, that Ember’s genetic heritage will endure, and that her life, though short, will have been worthwhile."

Some criticize the zoo for deciding to kill Ember. "Appears to be another example of a British zoo which doesn't value the lives of those in their care," Daniel Allen, a policy advisor for the UK Wild Otter Trust, wrote on Twitter.

But the zoo pointed out that it is accountable to the Zoo Licensing Act. "Wolves are classified as a category 1 dangerous animal," the spokesperson said. "Under the Zoo Licensing Act, if a category 1 animal escapes, you have no option but to euthanize. A decision which, as you can imagine, was devastating."